(a) Field of the Invention
The present invention relates to a lens system for facsimile and, more specifically, to a lens system which has performance required for facsimile and can be manufactured at low cost.
(b) Description of the Prior Art
It is required for lens systems for facsimile to have high OTF in combination with an aperture efficiency of 100%. Speaking more concretely, lens systems for facsimile must generally have OTF of 40 lines/mm for both paraxial and offaxial rays, especially OTF for offaxial rays being important. In order to assure an aperture efficiency of 100%, lens systems for facsimile must further have large diameters for favorably correcting coma. Furthermore, it is preferable to maintain spherical aberration a little curved toward the negative side at the maximum numerical aperture in order to match best focused plane as precisely as possible between paraxial and offaxial rays. On the other hand, it can be considered to design such lens systems as of the Tesser type for making them manufacturable at low cost. Though Tesser type lens systems have heretofore been developed in large numbers as photographic camera lens systems, there are available no lens systems which have large film sizes and high OTF. Further, lens systems designed for photographing object at infinite distance cannot be used as lens systems for facsimile since lens systems for facsimile are used for photographing objects at finite distances. Furthermore, lens systems for facsimile are used with a flat plate placed in front of the material to be photographed or light receiving surface and said plate produces astigmatism. From this viewpoint also, general Tesser type lens systems cannot assure high OTF. In addition, lens systems for facsimile must have an aperture efficiency of 100% and are different, in this point, from general lens systems for photographic cameras.